The present invention relates to a developing device for an image forming apparatus and having a relatively hard first developing roller formed with fine N-S magnetic poles on the periphery thereof, and a relatively soft second developing roller for conveying toner electrostatically transferred thereto from the first roller toward a photoconductive drum.
An electrophotographic system is customary with a copier, facsimile apparatus, printer or similar image forming apparatus. This kind of apparatus includes a photoconductive element or image carrier, and a developing device adjoining the element. A latent image is electrostatically formed on the photoconductive element. The developing device develops the latent image by depositing toner thereon in accordance with the potential distribution of the latent image. For the development, there are available two different systems, i.e., an S-NSP system using a relatively soft developing roller, and a .mu.-ISP system using a relatively hard developing roller. However, the problem with these conventional systems is that the toner is partly charged to polarity opposite to expected polarity. The oppositely charged toner forms black spots on the background of a recorded image and thereby lowers the image quality.
To obviated the problem attributable to the oppositely charged toner, use may be made of a magnetic first developing roller made of a hard material, and a second developing roller made of a soft material, as proposed in the past. Magnetic toner is magnetically deposited on the first roller, and then electrostatically transferred to the second roller which is so biased as to cause such toner transfer to occur. However, even such an improved scheme is apt to destroy the photoconductive layer of the drum, to bring about defective or unclear images, and to result in the need for an expensive high-tension power source and special equipment for insulation.